1. Field of the Invention
The present invention relates to an anode for lithium-ion secondary battery including an anode active material layer allowed to insert and extract lithium ions, a lithium-ion secondary battery using the same, and a power tool, an electric vehicle and an energy storage system using the same.
2. Description of the Related Art
In recent years, small electronic devices typified by portable terminals and the like have been widely used, and further size and weight reduction and longer life of the electronic devices have been strongly demanded. Accordingly, as power sources, batteries, in particular, small and lightweight secondary batteries allowed to obtain a high energy density have been developed. Recently, applications of such secondary batteries to not only small electronic devices but also large electronic devices typified by vehicles have been studied.
In particular, lithium-ion secondary batteries utilizing insertion and extraction of lithium ions for charge-discharge reactions holds great promise, because the secondary batteries are allowed to obtain a higher energy density than lead-acid batteries or nickel-cadmium batteries.
The lithium-ion secondary battery includes a cathode, an anode and an electrolytic solution. The anode includes an anode active material layer, and the anode active material layer includes an anode active material involved in charge-discharge reactions.
As the anode active material, a carbon material is widely used. However, since a further improvement in battery capacity has been demanded recently, it is considered to use silicon. The theoretical capacity of silicon (4199 mAh/g) is much higher than the theoretical capacity of graphite (372 mAh/g), so a significant improvement in battery capacity is expected.
However, when silicon is used as the anode active material, the anode active material swells and shrinks severely to easily cause a crack mainly in proximity to a surface thereof. When the anode active material is cracked, a high-reactive newly-formed surface (an active surface) is formed to cause an increase in a surface area (a reactive area) thereof. Therefore, the decomposition reaction of an electrolytic solution occurs on the newly-formed surface, and the electrolytic solution is consumed to form a coating film derived from the electrolytic solution on the newly-formed surface. Accordingly, cycle characteristic and initial charge-discharge characteristics which are important characteristics of the lithium-ion secondary battery easily decline.
Therefore, to improve battery characteristics such as cycle characteristics, various configurations of the lithium-ion secondary batteries have been studied.
More specifically, to improve cycle characteristics and safety, for example, as described in Japanese Unexamined Patent Application Publication No. 2001-185127, silicon and amorphous silicon dioxide are simultaneously deposited by a sputtering method. To obtain high battery capacity and high safety performance, for example, as described in Japanese Unexamined Patent Application Publication No. 2002-042806, electron-conductive material layers (a carbon material) are arranged on surfaces of silicon oxide particles. To improve high rate charge-discharge characteristics and cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2006-164954, an anode active material layer includes silicon and oxygen, and is formed so as to have a larger oxygen ratio on a side close to an anode current collector. To improve cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2006-114454, an anode active material layer includes silicon and oxygen, and is formed so that the average oxygen content in the whole anode active material layer is 40 at % or less and the average oxygen content on a side close to an anode current collector is larger. In this case, a difference between the average oxygen content on the side close to the anode current collector and the average oxygen content on a side far from the anode current collector is within a range of 4 at % to 30 at % both inclusive.
Moreover, to improve initial charge-discharge characteristics and the like, for example, as described in Japanese Unexamined Patent Application Publication No. 2009-070825, a nano-composite including a Si phase, SiO2, and a MyO metal oxide is used. To improve cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2008-282819, powdery SiOx (0.8≦x≦1.5, with a particle diameter range=1 μm to 50 μm) and a carbonaceous material are mixed and fired for 3 to 12 hours at 800° C. to 1600° C. To shorten an initial charge time, for example, as described in International Publication No. WO2007/010922, an anode active material represented by LiaSiOx (0.5≦a−x≦1.1 and 0.2≦x≦1.2) is used. In this case, lithium is evaporated on an active material precursor including silicon and oxygen. To improve charge-discharge cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2008-251369, the composition of silicon oxide is controlled so that the molar ratio of the amount of oxygen to the amount of silicon in an anode active material body is within a range of 0.1 to 1.2 both inclusive and a difference between the maximum value and the minimum value of the molar ratio of the amount of oxygen to the amount of silicon in proximity to an interface between the anode active material body and a current collector is 0.4 or less. To improve load characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2008-177346, a lithium-containing porous metal oxide (LixSiO: 2.1≦x≦4) is used.
Further, to improve charge-discharge cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2007-234255, a hydrophobic layer of a silane compound, a siloxane compound or the like is formed on a thin film including silicon. To improve cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2009-212074, conductive powder formed by coating a surface of silicon oxide (SiOx: 0.5≦x<1.6) with a graphite coating film is used. In this case, on Raman spectroscopy analysis, the graphite coating film develops broad peaks at 1330 cm−1 and 1580 cm−1 Raman shift, and an intensity ratio I1330/I1580 is 1.5<I1330/I1580<3. To improve a battery capacity and cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2009-205950, powder including 1 to 30 mass % of particles with a structure in which a microcrystal of silicon (with a crystal size of 1 nm to 500 nm) is dispersed in silicon dioxide is used. In this case, in a particle size distribution by a laser diffraction/scattering type particle size distribution measurement method, the 90% accumulated diameter (D90) of the power is 50 μm or less, and the particle diameters of the particles are smaller than 2 μm. To improve cycle characteristics, for example, as described in Japanese Unexamined Patent Application Publication No. 2009-076373, silicon oxide (SiOx: 0.3≦x≦1.6) is used, and an electrode unit is pressurized with a pressure of 3 kgf/cm2 or over during charge and discharge. To improve overcharge characteristics, overdischarge characteristic and the like, for example, as described in Japanese Patent No. 2997741, an oxide of silicon with a silicon-oxygen atomic ratio of 1:y (0<y<2) is used.